1. Field of the Invention
The present invention generally relates to surface acoustic wave (SAW) filters, and more particularly to a ladder-type SAW filter suitable for an RF (Radio Frequency) filter provided in pocket and mobile telephones, such as automobile phone set and portable phones.
2. Description of the Prior Art
In Japan, an automobile phone or portable phone system has a specification in which a transmission frequency band is .+-.8.5 MHz about a center frequency of 933.5 MHz. The ratio of the above transmission band to the center frequency is approximately 2%.
Recently, SAW filters have been employed in automobile phone or portable phone systems. It is required that the SAW filters have characteristics which satisfy the above specification. More specifically, it is required that the pass band width is so broad that 1) the ratio of the pass band to the center frequency is equal to or greater than 2%, 2) the insertion loss is small and equal to 5 dB-2 dB, and 3) the suppression factor is high and equal to 20 dB-30 dB.
In order to satisfy the above requirements, SAW filters are substituted for conventional transversal filters. Generally, SAW elements are so connected that a ladder-type filter serving as a resonator is formed.
FIG. 1 is an equivalent circuit of a SAW filter disclosed in Japanese Laid-Open Patent Publication No. 52-19044. A SAW filter 1 shown in FIG. 1 comprises a SAW resonator 3 in a series arm 2, and a SAW resonator 5 in a parallel arm 4. The equivalent parallel capacitance C.sub.OB of the resonator 5 in the parallel arm 4 is larger than the equivalent parallel capacitance C.sub.OA of the resonator 3 in the series arm 2.
The SAW filter 1 shown in FIG. 1 has a characteristic shown in FIG. 2. A curve 6 shows an attenuation quantity v. frequency characteristic of the SAW filter 1. As indicated by arrows 7 shown in FIG. 2, the suppression factor increases as the equivalent parallel capacitance C.sub.OB increases. However, as the equivalent parallel capacitance C.sub.OB increases, the band width decreases, as indicated by arrows 8, and the insertion loss increases, as indicated by an arrow 9. Hence, the characteristic deteriorates, as indicated by a broken line 10. When trying to obtain a suppression factor equal to or larger than 20 dB, the band width is decreased so that the ratio of the pass band to the center frequency is equal to or smaller than 1%, and does not satisfy the aforementioned specification of the 800 MHz-band radio systems.